The invention relates to holographic media containing specific photopolymers, a process for the production thereof, and unsaturated glycidyl ether acrylate urethanes as writing monomers which are suitable for the preparation of photopolymers.
Photopolymers—i.e. materials which polymerize by light—are very widely used, such as, for example, in the coating of generally flat substrates, such as paper and wood in the furniture, film, parquet or printing industry. Moreover, there are many further special applications. Classically used materials are esters of (meth)acrylic acid, polyester acrylates, epoxy acrylates and urethane acrylates. A fairly seldom described class of substances comprises the glycidyl ether acrylate urethanes. EP 44352 teaches, for example, about the use thereof in dental compound compositions containing alkylglycidyl ether methacrylate urethanes—and phenylglycidyl ether methacrylate urethanes. JP 118475 describes the production of plastics lenses based on halogenated methylphenyl glycidyl ether acrylate urethanes. The production of optical elements having polarizing properties is taught by JP 2006243416, in which nonpolymerizing liquid crystalline materials are combined with polymerizing unsaturated materials, such as esters of unsaturated acids with aliphatic polyols and amides of unsaturated acids with aliphatic polyamines, and the abovementioned polyester acrylates, epoxy acrylates and urethane acrylates. In an embodiment described, a phenylglycidyl ether acrylate urethane based on hexamethylene diisocyanate is used for this purpose. These formulations are liquid and must be polymerized with light immediately after application.
Specific photopolymers are suitable for the production of volume holograms, which are structured by means of exposure to coherent radiation sources, such as, for example, laser radiation. A three-dimensional structure forms in the photopolymers, which structure can be described in general by a regional change of the refractive index in the material. A hologram is therefore an object which contains a periodic, spatial modulation of the refractive index. The optical function which such a hologram performs, for example representing a three-dimensional image or being capable of being used as a diffractive optical element, depends on the specific exposure.
For the use of photopolymers as a carrier of holograms for optical applications in the visible range, colourless or only very faintly coloured materials having a high diffraction effect are as a rule required after the exposure. Since the beginning of holography, silver halide films, in particular those having high resolution, have been used for this purpose. Dichromate gelatin (DCG), dichromate salt-containing gelatin films or mixed forms of silver halide and DCG are also used. Both materials require a chemical aftertreatment for the formation of a hologram, which gives rise to additional costs for industrial processes and makes it necessary to handle chemical developer solutions. Moreover, wet chemical processes have the disadvantage of causing swelling under the action of the developer solutions. During the subsequent drying, shrinkage of the film occurs. This generally leads to colour shifts and irregularities in the hologram imaging, which is undesired. Although this wet chemical process can be realized technically, experience in the holographic industry has shown that high levels of waste due to the complexity and required precision of the process leads to unacceptable high costs.
Various approaches were adopted for replacing the above materials. U.S. Pat. No. 4,959,284 (Dupont) describes photopolymers which consist, inter alia, of a thermoplastic, such as polyvinyl acetate, cellulose acetobutyrate or polymethyl methacrylate-styrene-copolymers, soluble in organic solvents, a photoinitiator and at least one vinylcyclopropane derivative. Moreover, EP352774A1 (Dupont) describes other monomers containing vinyl groups, such as N-vinylpyrrolidone, phenoxyethyl acrylate and acrylates of triols, such as trimethylolpropane (TMPTA) and ethoxylated trimethylolpropane (TMPEOTA) or other acrylates or acrylamides.
Photopolymers which are contained not from thermoplastics but from crosslinked polymers have also been recently described: U.S. Pat. No. 6,103,454 (InPhase) describes a polyurethane matrix comprising polymerizable components, such as 4-chlorophenyl acrylate, 4-bromostryrene and vinylnaphthalene. These formulations were developed for holographic data storage, a holographic application in which many holograms which are very weak holograms readable only by means of electronic detectors are written and read out. Holographic media based on such a material are not suitable for the production of holograms visible to the eye.
It was now an object to provide holographic media which develop at room temperature only under coherent radiation and do not require any thermal or chemical aftertreatment. Furthermore, it was the object to provide for this purpose polymerizable writing monomers which are particularly suitable for the preparation in such photopolymer compositions. It was important to find polymerizing writing monomers which can be particularly readily dissolved in the binder.